MARYLAND GEOLOGICAL SURVEY 73 



Character of Materials. 

 The materials composing the Calvert formation are as a whole quite 

 uniform. They consist of clay, marl and diatomaceous earth. Through- 

 out its entire extent the formation is abundantly supplied with fossils. 

 The diatomaceous earth is greenish-blue when fresh, and weatliers to 

 a liglit buff when exposed to the air. It is found in the lower 

 portions of the formation, although diatoms are abundant at many 

 other horizons. The clay and marl are also dark brown to bluish-green 

 when fresh and change to various tints of buff on exposure to the 

 weather. The fossils frequently are concentrated in great bands which 

 are remarkably constant throughout the formation. (Plate II.) 



Strati grapJiic Relations. 

 The Calvert formation lies unconformably on the eroded edges of 

 the Nanjemoy. This unconformity is in the nature of an overlap and 

 is most easily seen along the cast bank of the Patuxent River in the 

 vicinity of Lyons Creek. It may also be seen in one of the southern 

 tributaries of Lyons Creek, near the place where the highway leading to 

 Chaney is crossed by the Chesapeake Beach Eailroad. In both of these 

 localities a thin stratum of silicious sandstone bearing fossils is clearly 

 discernible. This lies in the body of the Calvert formation and about 

 2 feet above its contact with the underlying Nanjemoy. In the last 

 locality mentioned a waterfall has developed on this hard stratum 

 (Plate I). Above, the Calvert formation lies unconformably beneath 

 the Choptank formation. 



Suh-Divisioiis. 



Within the county borders the Calvert formation falls into two divi- 

 sions which are known as the Fairhaven diatomaceous earth and the 

 Plum Point marls. 



Fairhaven Diatomaceous Earth. — This member lies at the base 



of the Calvert formation and is characterized by the presence of a large 



proportion of diatoms imbedded in a very finely divided quartz matrix. 



Calcareous material is present in this bed only in very small amounts. 



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